They are known cassette gaskets of the above type, suitable to be used in working vehicles, such as tractors or earth-moving machines, under extremely severe operating conditions, characterized for example by:                intermittent operation of the motor, i.e. with continuous passages from a start condition to a stop condition and vice versa;        continuous back and forth movements of the vehicles, with angular movements in opposite directions of the rotating members on which the cassette gaskets are applied;        partial immersion of the vehicles in mud;        adverse weather conditions; and        relatively high circumferential speed of the rotating members on which the cassette gaskets are applied.        
An example of a cassette gasket of the known type, having an annular shape around an axis, is shown in the patent application DE-C-19539056 and comprises:                a first support ring having a first annular fixing portion, adapted to be connected in use to the rotating member;        a second support ring having a second annular fixing portion, radially more external than the first fixing portion and adapted to be connected in use to the fixed member;        a first annular elastomeric element carried by the first support ring and defining two annular sealing lips cooperating with the second fixing portion to counter, in use, the passage of external contaminants towards the ambient containing the lubricant; and        a second annular elastomeric element carried by the second support ring and defining a further annular sealing lip cooperating with the first fixing portion of the first support ring to counter, in use, the passage of lubricant to the outside.        
The second elastomeric element further comprises a further sealing lip adapted to cooperate with the first fixing portion of the first support ring to prevent external contaminants from reaching the adjacent lubricant sealing lip.
The aforesaid gaskets, although being functionally effective, are susceptible of further improvements, in particular as regards the number of working hours they can guarantee under the aforesaid severe operating conditions.